1. Field of the Invention
This invention relates to vibration-reducing means employed in conjunction with flexible annular seal structures in gas turbine engines.
2. Description of the Prior Art
In modern gas turbine engines, major components of the engine are designed to be assembled and disassembled in modules. During engine operation, these modules expand and contract both axially and radially because of temperature differences in different sections of the engine. The means of attachment between these modules must be flexible and capable of dimensionally absorbing this expansion and contraction. In addition, at some points of attachment, these modules define boundaries between coannular fluid flowpaths that are pressurized to different levels. To prevent leakage between these fluid flowpaths, the means of attachment must maintain an effective sealing relationship between modules and must also dimensionally absorb expansion and contraction.
A particular type of seal structure that accomplishes these functions and has been successfully employed as an attachment means between modules in gas turbine engines is known as a "floating fishmouth seal." Floating fishmouth seals are comprised of two sections, a male section and a female section. The male section is an annulus that extends into an annular inlet in the female section. The female section is generally comprised of two annular arms that are joined together at one end and diverge at the other end thereby forming the annular inlet. The floating fishmouth seal will accommodate expansion and contraction in both the radial and axial directions, because the male section can move both radially and axially within the annular inlet in the female section and still maintain a sealing contact. This floating fishmouth seal construction has been successfully employed at various locations in gas turbine engines, and the fishmouth seal structure is generally well known to those skilled in the art.
While the capability of a fishmouth seal to absorb dimensional variations is very desirable in gas turbine engine applications, the relative movement of the male and female sections of the fishmouth seal tends to encourage vibration within the sealing structure. Any propensity to vibrate can be exaggerated under the extreme pressure and high rotational velocities incurred during gas turbine gas operation. These conditions can be particularly troublesome in flexible sealing structures in which there are widely temperature differences and in which large dimensional variations are present. Additionally, the sealing structure must prevent gas leakage between regions of widely varying pressure. In particular, it is necessary that the sealing structure provide an effective seal between the relatively low pressure gas flowpath products and the relatively high pressure cooling air flowing through a surrounding cooling plenum. The large pressure differences between these two gas flows often cause the seal to become self-excited by air leakage across the sealing structure. This self-excitation is referred to by those skilled in the art as aeroelastic instability. A condition of aeroelastic instability often leads to excessive and uncontrolled vibration.
Uncontrolled vibration at this location can lead to rapid fatigue failure or cracking of the combustor or turbine members or the sections of the seal structure itself. Failure in the sealing structure can result in small pieces of the seal being blown into the gas flowpath of the engine causing excessive damage to downstream parts. Therefore, means for preventing excessive vibration in the sealing structure are necessary to maintain successful engine operation.